Section 10.2.1. Crack Growth Resistance and Fracture Toughness
The material properties used for the selection criteria must be
obtained for conditions that correspond to those expected in the structural
usage environment. Crack growth
resistance as expressed in the da/dN
data should be obtained from tests conducted using thickness similar to the anticipated structure applications and in
similar environments. Some alloys are
quite susceptible to corrosive media such as may be experienced in
aircraft fuel bays or during operation near salt water. Effects of these variables are shown in Figure 10.2.2 [Circle & Conley, 1980].
Figure 10.2.2. Illustration of Effects of Environment on
Crack Growth Rates [Circle & Conley, 1980]
For ease of application in the design process, the crack
propagation data is usually described by an empirical relationship, such as the
Forman equation, given as:
Kc - fracture toughness
stress-intensity factor range
C, n -
constants dependent on material, obtained from curve fitting techniques
It may be necessary to model the data in several parts over the
DK range of interest in
order to achieve adequate representation.
Ekvall, et al. 
presents a method for evaluation of weight savings due to the usage of advanced
materials. The utilization of materials
having improved damage tolerance characteristics as evidenced by a higher
allowable stress value was shown to effect a weight savings from 1-3 percent
for an improvement in allowable stress of 10-25 percent.
Simenz and Guess  discusses material properties and
characteristics of some new materials based on obtaining high strength with
good durability and damage tolerance properties. This is mentioned to make the reader aware of current efforts to
improve structural materials. Goals
stated in this report are to increase the static strength, decrease the crack
growth, and increase the temperature capability of aluminum alloy.